Efficacy trials show that a dissonance-based selective prevention program that decreases pursuit of the unrealistic thin-ideal in high-risk young women with body image concerns reduces eating disorder risk factors (e.g., body dissatisfaction, negative affect), eating disorder symptoms, functional impairment, and future eating disorder onset relative to alternative interventions and assessment-only controls, with some effects lasting 3 yrs (Becker et al., 2005; Mitchell et al., 2007; Stice et al., 2000, 2006, 2008b, 2011c). An effectiveness trial found that when high school clinicians recruit students and deliver the program that it produces greater reductions in outcomes relative to educational brochure controls, including reduced eating disorder symptoms through 3-yr follow-up (Stice et al., 2009b). Larger effects are emerging from an effectiveness trial of an enhanced-dissonance version of this program with college clinicians (Prelim Studies). These trials confirm the effectiveness of this intervention, but reveal it can be difficult to identify and recruit clinicias at high schools and colleges with the time and expertise to capably deliver this prevention program. One solution to this dissemination barrier is to train college students in peer leader programs to recruit students and deliver the program, as peer-led interventions appear promising, sometimes producing larger effects than clinician-led versions of the intervention (Botvin et al., 1984; Rhee et al., 2011). Use of peer leaders could markedly extend the reach and sustainability of this program. In a pilot trial, peer-led groups of the enhanced-dissonance program produced significantly greater pre-post reductions in thin-ideal internalization, body dissatisfaction, dieting, negative effect, and eating disorder symptoms relative to educational brochure controls (M d = .76), producing effects similar to those from clinician-led groups. Effects compare favorably to those from clinician- led groups in our college effectiveness trial (M d = .66) and from uncontrolled studies of a peer-led version of the dissonance program adapted for sororities (M d = .46; Becker et al., 2010; Perez et al., 2010). We propose to conduct a large multi-site effectiveness trial of peer-led enhanced-dissonance groups, wherein 480 female college students with body image concerns will be randomized to peer-led groups, clinician-led groups, or an educational video control condition. Aim 1: test whether participants who complete peer-led groups show greater reductions in eating disorder risk factors, symptoms, eating disorder onset, and functional impairment over 3-yr follow-up than educational video controls and reductions similar to those who complete clinician-led groups. Aim 2: test whether fidelity and competence ratings are similar for peer-led vs. clinician-led groups. Aim 3: test whether reductions in thin-ideal internalization mediate the intervention effects on other downstream outcomes and examine moderators of intervention effects (e.g., elevated baseline thin-ideal internalization) for peer- and clinician-led groups. Aim 4: test whether cost savings and incremental cost-effectiveness will be greater for peer-led groups vs. clinician-led groups relative to the educational video control condition.